Supernumerary Carpal Bones

► [Extra-ossicles throughout the carpus]

In addition to the normal eight carpal bones, more than 20 accessory ossicles disseminated throughout the carpus have been described (O'Rahilly 1953; Poznanski and Holt 1971). Their incidence varies in different series from 0.3% to 1.6% in the general population (O'Rahilly 1953; Bogart 1932). These extra carpal ossicles produce no symptoms or signs in the majority of cases, and they are usually of no clinical significance. The os centrale, an extra-ossicle located between the scaphoid, capitate, and trapezoid (Fig. 6.53) (Gerscovich and Greenspan 1990), is constantly present as a cartilaginous nucleus in the developing embryo at about 6 weeks of gestation and usually fuses with the scaphoid shortly thereafter (Tardif et al. 1998). Phylogenetically it is a remnant of the central row of carpals present in more primitive animals. Occasionally the os centrale persists into adult life as an isolated anatomical variant in the shape of a well-marginated independent bone or as an incompletely separated bony element with smooth contours (Greenspan and Gerscovich 1993). Although usually asymptomatic, the os centrale has

Accessory Ossification Centre Scaphoid

Fig. 6.53. Os centrale carpi in a 21-year-old man with a 3-year history of pain in his left wrist. Note the presence of bilateral ossicles overlapping the distal pole of the scaphoid, which is consistent with os centrale carpi. The left accessory ossicle is slightly sclerotic due to histologically proven osteonecrosis. (From Abascal et al. 2001

Fig. 6.53. Os centrale carpi in a 21-year-old man with a 3-year history of pain in his left wrist. Note the presence of bilateral ossicles overlapping the distal pole of the scaphoid, which is consistent with os centrale carpi. The left accessory ossicle is slightly sclerotic due to histologically proven osteonecrosis. (From Abascal et al. 2001

been associated with painful clicking (Adolfsson 2000) and osteonecrosis (Lane et al. 1990; Abascal et al. 2001). The major clinical concerns, however, remain its differentiation from a scaphoid fracture in the acute stage (Yang et al. 1994) and the possible superimposition of osteoarthritic changes in the chronic stage (Lane et al. 1990). An acute scaphoid fracture shows noncorticated sharp or irregular margins, whereas an old scaphoid fracture may show cyst-like formation or patchy decalcification along the fracture line (Abascal et al. 2001). An expanded differential diagnosis also includes the bipartite scaphoid and the rare instances of dysplasia epiphysealis hemimelica involving the hand. More often than in normal individuals, the os centrale occurs in patients with such disorders as hand-foot-uterus syndrome (OMIM 140000), Holt-Oram syndrome (OMIM 142900), oto-palato-digital syndrome (OMIM 311300), and Larsen syndrome (OMIM 150250,245600). It has also been reported in two sisters with an association of cleft soft palate, severe oligodontia, bilateral conductive deafness due to stapes fixation, short halluces, and tarsal coalition (Gorlin-Schlorf-Paparella syndrome, OMIM 216300) (Gorlin et al. 1971). Thus, the radiographic identification of the os centrale may be helpful in the diagnosis, and especially in the recognition of ill-defined syndromes or of disorders with subtle clinical manifestations. The os triangulare is a relatively common extra-ossicle located in the triangular cartilage of the wrist just distal to the ulnar styloid (Akpinar et al. 1999). This ossicle has its own cartilaginous nucleus during fetal life, but this usually disappears before birth. It occurs as a normal anatomical variant and is devoid of any diagnostic relevance.

In addition to the ossicles mentioned above, there are several other extra bones that can occur in the carpus in association with malformation syndromes. Accessory carpal bones are sometimes found in the distal row in brachydactyly A-1, diastrophic dysplasia, Ellis-van Creveld syndrome, Larsen syndrome, and oto-palato-digital syndrome (Hoefnagel and Gerald 1966). In Larsen syndrome (OMIM 150250) a considerable increase in the number of the carpals scattered in a random fashion is often associated with bizarre changes of their shape. Additional features in the hands include end expansion of the metacarpals and proximal phalanges, short distal phalanges, premature fusion of the epiphysis to the shaft in the distal phalanx of the thumb, and pseu-doepiphyses (Latta et al. 1971). The features of oto-palato-digital syndrome (OMIM 311300) include supernumerary carpal bones, often distributed along the ulnar side of the wrist; partial or complete fusion (especially capitate-hamate); and malpositioning of individual bones (most typically, a rotation of the capitate in the horizontal plane). Additional manifestations include short and broad distal phalanges, with premature epiphyseal fusion; 2nd finger clin-odactyly; and proximal elongation of the ulnar side of the 2nd metacarpal, with a prominent accessory ossification center. Overall, the hand manifestations in this disorder are highly characteristic (Langer 1967). In mirror-imagepolydactyly (OMIM 135750), a rare condition involving absence of radius and tibia with complete duplication of ulna and fibula and polysyndactyly, the carpal bones are absent on the radial side and duplicated on the ulnar side (Hersh et al. 1995).

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